The transmission of soliton pulses or "solitons" in the portion of an optical fiber that has abnormal dispersion is a known phenomenon. Solitons are pulse signals of sech.sup.2 form. With a pulse of this form, the non-linearity of the corresponding portion of the fiber compensates for the dispersion in the optical signal. Soliton transmission is modelled in known manner by means of the non-linear Schrodinger equation.
Various effects put a limit on the transmission of such pulses, such as the jitter induced by interaction of solitons with the noise present in a transmission system, as described for example in the article by J. P. Gordon and H. A. Haus, published in Optical Letters, Vol. 11, No. 10, pages 665-667. This effect which is known as the "Gordon-Haus effect" or as "Gordon-Haus jitter" puts a theoretical ceiling on the quality of soliton transmission or on its data rate. Another limit is imposed by amplitude fluctuations due to amplifier noise.
To overcome these limits, it is possible to use synchronous modulation with soliton signals, by means of semiconductor modulators. That technique puts an intrinsic limit on the data rate of a soliton link, because of the upper limit on the passband of semi-conductor modulators. Proposals have also been made for systems having sliding guiding filters enabling the jitter of transmitted solitons to be controlled, see for example EP-A-0 576 208.
For the purpose of regenerating the signal on a line, proposals have also been made to use the Kerr effect in synchronous amplitude or phase modulators of the interferometer type. These techniques require high frequency components to be used, thus demonstrating the advantage of all-optical approaches. In terms of cost, this approach is limited firstly by the number of in-line modulators that are required, together with their associated electronics, and secondly, in the intended undersea applications, by the need to have components of appropriate quality rating.
Finally, to regenerate soliton signals, Proposals have been made to use saturable absorbers. That solution is subject to three limitations. Firstly, the time response of saturable absorbers does not appear to be fast enough for high data rate applications; secondly saturable absorbers do not control jitter as effectively as synchronous modulation; and finally saturable absorbers do not appear to be compatible with propagation using wavelength multiplexing, because of the lack of synchronism between channels.
An article by D. Atkinson et al., Optics Letters, Vol. 19, No. 19, pages 1514-1516 decribes a theoretical simulation of a soliton signal transmission with quantum-well saturable absorbers and spectral filtering. In that article, it is shown that it is theoretically possible to transmit non-multiplexed soliton signals over a distance of 9000 km with regenerators that are regularly spaced apart at intervals of 100 km. Each regenerator comprises an amplifier, a multiple quantum well (MQW) saturable absorber, and a narrow band filter. The article specifies that the absorption spectra of MQWs limit their usefulness in wavelength-multiplexed systems. It is only suggested that certain saturable absorber materials could, in principle, present a special wavelength and thus be compatible with wavelength multiplexing.
An article by R. J. Essiambre and G. P. Agrawal, J. Opt. Soc. Am. B/Vol. 12, No. 12, pages 2420-2425 proposes a theoretical description of a short soliton signal transmission system under quasi-adiabatic propagation conditions. The simulated transmission system has regenerators including a limited-bandwidth amplifier, a fast saturable absorber, and where appropriate, a frequency filter. The article explains that the saturable absorber makes it possible to reduce resonance instabilities, and prevents secondary solitons being produced. More precisely, the article proposes compensating Raman diffusion within each pulse, which gives rise to the frequency of a soliton being shifted downwards, by an upward frequency shift in each regenerator. That solution cannot be transposed to wavelength-multiplexed transmission systems, given the importance of narrow bandwidth for the amplifiers, and the relationships between soliton frequency and spacing between repeaters.
Also, to increase the data rate of optical fiber transmission systems using soliton signals, proposals have also been made to use wavelength division multi-plexing (WDM). Under such circumstances, it is believed to be advantageous to use sliding guiding filters of the Fabry Perot type, which are fully compatible with wavelength-multiplexed signals. In contrast, the use of synchronous modulators or of saturable absorbers for regenerating wavelength-multiplexed soliton signals is problematic because of the difference in group velocity between the signals in the various channels.
An article by E. Desurvire, O. Leclerc, and O. Audouin, Optics Letters, Vol. 21, No. 14, pages 1026-1028 describes a scheme for allocating wavelengths that is compatible with using synchronous modulators. That article proposes giving wavelengths to different channels of the multiplex in such a manner that for repeaters at given spacing, the signals in the various channels, or more precisely the bit times of the various channels of the multiplex, are substantially synchronized on reaching the repeaters. This makes synchronous in-line modulation of all of the channels possible at given intervals by means of discrete synchronous modulators. A more detailed article describes the various ways in which wavelengths can be allocated in the more general case, i.e. with or without dispersion slope compensation (O. Leclerc, E. Desurvive, and O. Audouim, Optical Fiber Technologies, Vol. 3, No. 2, April 1997, Academic Press, 1997). The general technique of allocating wavelengths in the multiplex is also described in French patent application 96/00732 of Jan. 23, 1996 (corresponding to U.S. Pat. No. 5,801,862) in the name of Alcatel Submarine Networks.
Other methods or apparatuses, for example those described in French patent application 97/01476 of Feb. 10, 1997 (corresponding to U.S. patent application Ser. No. 09/155,901) in the name of Alcatel Alsthom Compagnie Generale d'Electricite make it possible to achieve the same result, i.e. bit time synchronization of the various channels in the multiplex at certain positions along the transmission system. In that patent application, it is proposed to use delay lines or equivalent devices to resynchronize the channels of the multiplex.